1. Field of the Invention
The present invention relates to an image-pickup apparatus, or more particularly, an image-pickup apparatus capable of substantially reproducing an image obtainable with a wide dynamic range.
2. Description of the Related Art
Various types of image-pickup apparatus for electrically producing an object image have been proposed in the past. In a digital still camera that is one type of image-pickup apparatus, a solid-state image-pickup device converts a light image, which is converged on a surface of the solid-state image-pickup device through a lens, into electric information and thus produces an image.
The solid-state image-pickup device incorporated in the digital still camera can produce many pixels at present and can therefore produce an image of a higher resolution. However, since the image-pickup device can offer only a very narrow dynamic range, a resultant image suffers from a narrow latitude (a range of reproduced brightness levels from a highlight to a shadow).
What is essential to reproduce an image despite the narrow dynamic range is an automatic exposure setting (AE) facility that properly determines a condition for exposure optimal to a photographic scene (exposure time (shutter speed) or an f-number). Some proposals have been made for a technology of reproducing an image using the AE facility in the past.
As an example of such a technology, Japanese Patent Unexamined Publication No. 6-38092 describes a technology concerning a video camera that can properly determine a condition for exposure and a photometry method employed in the camera. More particularly, according to the technology, when a value calculated by integrating luminance values that are specified in image information acquired for a photometric period within a horizontal scanning period is judged to fall within a predetermined range, photometry is performed based on the value. Furthermore, the patent unexamined publication describes that if the number of times by which the value calculated by integrating the luminance values is judged to fall outside the predetermined range exceeds a predetermined value, the condition for exposure is changed.
In order to acquire image information, which is obtainable with a wide dynamic range, from a solid-state image-pickup device suffering a narrow dynamic range, exposure is performed a plurality of times with an exposure level varied. This technology is well known.
The foregoing technologies will be described with reference to FIG. 19A, FIG. 19B, and FIG. 19C that show a photographic scene whose objects having a difference in luminance are exposed with an exposure level varied and which is thus exposed a plurality of times.
FIG. 19A shows a photographic scene having a figure, which is a dark major object, located nearly in the center of a background that is bright scenery. In order to pick up an image of the scenery using a solid-state image-pickup device that suffers a narrow dynamic range, exposure is performed with an exposure level lowered so that the background will appear more distinctly as shown in FIG. 19B. Moreover, exposure is performed with the exposure level raised so that the figure will appear more distinctly as shown in FIG. 19C. The background portion of the image shown in FIG. 19B and the figure portion of the image shown in FIG. 19C are combined in order to produce an image obtainable with a wide dynamic range. The image obtainable with a wide dynamic range shows both the background and figure distinctly.
Even when the foregoing technology of performing exposure a plurality of times is used to produce an image obtainable with a wide dynamic range, the AE facility plays a significant role in reproducing an optimal image. Specifically, when a scene which requires a wide dynamic range and in which a dark major object (figure) and a bright background coexist as shown in FIG. 19A, FIG. 19B, and FIG. 19C is imaged, the major object and background alike must be imaged while being exposed optimally.
Some technologies have been proposed for controlling photographing according to a photographic scene, which requires a wide dynamic range, so that both a major object and a background can be exposed optimally. For example, Japanese Patent Unexamined Publication No. 7-298142 describes an image-pickup apparatus that has a gray scale control facility. The gray scale control facility properly exposes both a major object and a background contained in a backlit photographic scene. To be more specific, the technology described in the patent unexamined publication requires a gray scale control means that switches timings, which set storing time of light information, according to whether a luminance value specified in image information indicates a high-luminance point or a low-luminance point.
Moreover, Japanese Patent Unexamined Publication No. 11-205661 describes a means that synthesizes a long-time exposure image produced under the predetermined condition for exposure, and a short-time exposure image produced while being exposed for a shorter exposure time. The means adjusts a ratio of exposure levels according to the position of a peak level point in a synthetic image.
However, according to the Japanese Patent Unexamined Publication No. 6-38092, if the number of times by which a value calculated by integrating luminance information falls outside a predetermined range exceeds a predetermined value, a condition for exposure is changed. Thereafter, photometry is performed in the same manner. As long as the scene shown in FIG. 19A, FIG. 19B, and FIG. 19C is concerned, after an initial condition for exposure is changed, it must be judged whichever of a bright point and a dark point should be subjected to photometry. Furthermore, according to the means, the condition for exposure under which either the bright point or dark point is imaged is optimized, and the condition for exposure under which the other point is imaged remains improper.
Moreover, according to the Japanese Patent Unexamined Publication No. 7-298142, in order to produce an image that enjoys excellent reproducibility, information concerning a photographic scene must be read from a solid-state image-pickup device or the like all the time. This results in an increase of power consumption. Furthermore, when photographing is performed a plurality of times with an exposure level varied, information of a bright point and information of a dark point must be read independently of each other. Based on the read information, control must be extended so that both the bright and dark points will be imaged properly all the time. This may lead to a larger amount of consumed power compared with an amount normally consumed power.
As a measure to be taken for lightening a load imposed on the AE facility, a means is conceivable. Namely, the AE facility is used to control exposure of either a major object or a background. The other of the controlled major object or background is exposed at a fixed exposure ratio. If an exposure ratio is thus fixed, the AE facility can be designed similarly to that adopted for a typical digital still camera.
However, a backlit state in which a major object and a background are distinguished from each other includes various cases. If brightness changes moderately between the major object and background, photographing may fail. Moreover, if a difference in brightness is as large as a difference in brightness between a dark indoor and a bright outdoor is, a scene may not be reproduced with an exposure ratio set to a value permissible for imaging of a normally backlit scene.
Furthermore, the condition for exposure under which one of bright and dark points is photographed may not be determined using the AE facility, and the condition for exposure under which the other point is photographed may not be determined using a fixed ratio. Instead, a means enabling a user to designate an exposure time and an exposure ratio is conceivable. However, a user may have difficulty in judging a proper exposure time or exposure ratio according to a difference in brightness. But for a certain experience or knowledge, it may be hard to judge what exposure time or exposure ratio is optimal. The means is therefore unpractical.
In addition, a means is conceivable for preparing a plurality of exposure settings in association with photographic scenes, and allowing a user to select any of the exposure settings. However, a user who does not have for a certain experience or knowledge may have difficulty in subjectively judging from a difference in brightness or the like what exposure setting is optimal for an actual photographic scene. The means cannot therefore be said to be practical.
According to the Japanese Patent Unexamined Publication No. 7-298142, when exposure is performed a plurality of times with an exposure level varied, a high-luminance point and a low-luminance point are specified manually in units of each of blocks into which an image field is divided. It is not clearly described how to automatically identify the high-luminance point and low-luminance point. Besides, how to identify the high-luminance point and low-luminance point after photographic scenes are changed is not clarified.
Moreover, according to the Japanese Patent Unexamined Publication No. 11-205661, the long exposure-time image and short exposure-time image are produced under the predetermined condition for exposure. If the levels of a signal representing an object do not fall within the range from the level of the signal representing the long exposure-time image to the level of the signal representing the short exposure-time image, an incorrect peak level may be detected. This poses a problem in that the ratio of exposure levels is not calculated properly.